The fuel flow rate to the burners of a gas turbine engine or the area of a variable exhaust nozzle is typically modulated to control, either directly or indirectly, the ratio of the average total pressure at a station in the engine to the average total pressure in the plane of the leading edge of the first cascade of airfoils in the engine, which is referred to herein as the inlet face plane. This first cascade may be stationary inlet guide vanes or rotating fan blades. The total pressures are measured directly by suitable probes which feed the pressure signals to means which calculate the pressure ratio and adjust the fuel flow to the burners in a scheduled manner. Correction factos and temperature and pressure limits and the like may also be taken into account to eliminate the effect of, for example, engine aging. One such control for a gas turbine engine is described in commonly owned U.S. Pat. No. 4,159,625 by W. B. Kerr, the inventor of the present application.
With some engine installations there is a total pressure gradient in the inlet face plane. This is generally caused by the air induction system to the engine which, when subjected to high "g" maneuver conditions, distorts the airflow distribution and pressure at the engine face. In engines where there is this distortion of the airflow, an accurate measurement of the average total pressure in the inlet face plane requires a large number of total pressure probes (perhaps three or four dozen) appropriately distributed in the inlet face plane. In those situations, controlling the engine using a direct measurement of the average total pressure in the inlet face plane involves a considerable amount of expensive hardware and undesirable instrumentation complexity.
Commonly owned U.S. Pat. No. 4,228,650 is of interest as regards the present invention for its teaching that if a device for directly measuring burner pressure (or other parameter) fails, a burner pressure signal can be synthesized or simulated. This is accomplished by a function generator which is fed other measured parameters and produces an output signal indicative of the ratio of the burner pressure to another engine parameter. The ratio is then multiplied by the other engine parameter, yielding a signal indicative of the synthesized burner pressure signal which is then passed to a control to substitute for the failed sensed parameter.